Controlled electrical doping of inorganic semiconductors has been key to the success of microelectronics. Methods to produce controlled electrical doping of thin organic semiconductor layers with a thickness in the range from a few nanometers to hundreds of nanometers, is expected to play an enabling role for the development of organic electronic devices such as organic light-emitting devices (OLEDs), organic thin-film transistors (OTFTs), organic photovoltaic devices (OPVs), organic photodetectors, organic memories, and any device containing an organic semiconductor in which electric charge is introduced or removed. Chemical and electrochemical doping are known methods for electrical doping traditional organic semiconductors.
Chemical doping (p[n]-type) is typically realized by mixing strong electron acceptor [donor] molecules within the bulk of an organic semiconductor layer to generate free holes [electrons]. The generated free carriers increase the conductivity and thus minimize ohmic losses through the organic semiconductor and in certain cases facilitate carrier injection and extraction by reducing the contact resistance. Furthermore, dopant molecules need to be immobilized within the bulk of the organic semiconductor to prevent diffusion of dopant molecules and degradation of the electronic properties of the devices.
Chemical doping of organic semiconductors is also typically realized through the coevaporation under vacuum of dopant and organic semiconductor molecules to form a thin organic semiconductor layer on a substrate. Alternatively, doped organic semiconductor layers can be processed by dissolving dopant and organic semiconductor molecules into organic solvents to form a solution from which a thin layer can be processed onto a substrate or any type of underlying layer. However, this method requires that the ionic species formed upon charge transfer reactions between organic semiconductor and dopant molecules remain soluble enough in order to avoid the precipitation of reactants. Although methods to produce thin doped organic semiconductor layers from solution in ambient conditions hold the promise to be more economical than those requiring material evaporation under vacuum, limited solubility of reactants can hinder the ability to process uniform high-quality doped-films that are suitable for organic electronic applications.
Polyoxometalates are a well-known class of transition metal-oxide nanocluster materials with various sizes for a very wide range of applications. Polyoxometalates are polyatomic ions typically consisting of transition metals such as vanadium (V), niobium (Nb), tantalum (Ta), molybdenum (Mo) and tungsten (W) in high oxidation states covalently linked through oxygen atoms forming a closed 3-dimensional framework with general formula {MOx}n, where M=Mo, W, V or Nb. Polyoxometalates display a very large range of framework structures such as Keggin, Dawson, Anderson, Lindqvist structures among many others. Of particular interest are frameworks comprising transition metal oxides and acidic hydrogen atoms linked to an element such as silicon (Si), phosphorous (P), arsenic (As), tungsten (W), among others typically chosen from the p-block of the periodic table, having a general formula Hk{XsMnOm}p, where X is an element referred to as heteroatom typically chosen from the p-block and located at the center of the framework, and M is a transition metal. Frameworks having this composition are known as heteropolyacids and are widely used as catalyst. Heteropolyacids with a ratio X/M=1/12 typically form frameworks having a Keggin structure and typically display good thermal stability, high acidity and are strong oxidants.
Phosphomolybdic acid (PMA), phosphotungstic acid (PTA) and silicotungstic acid are known heteropolyacids that have been used in organic electronic devices such as organic light-emitting diodes and organic photovoltaics as hole and electron transport layers, but not as dopants. A limitation of using polyoxometalates as dopants of solution processable organic layers is that their radical salts display limited solubility in commonly used organic solvents and precipitate in solution, thus preventing processing of uniform thin layers required for efficient organic electronic devices, and limiting the ability to control the degree of doping.
A need exists for improved methods and dopant/solvent combinations that allow controlled electrical doping of high-quality organic semiconductor layers for use with organic electronic devices.